Electrical detonator



29, 1959 D. D. TAYLOR ELECTRICAL DETONATOR Filed Aug. 4, 1953 IN VENTOR. DAN/EL DWIGHT TAYLOR BY H/S HTTO/ZNEYS. Haze/s, K/ECH, Fos TEE & HHRRIS United States Patent Altadena, Califl, assignor to Beck- South Pasadena, Calif., a cor- 'This invention relates to electrical detonators in "general and is directed specifically "to the problem of meeting the exacting requirements of modern weapons including guided missiles and various military devices equipped with automatic fuzes.

A current military specification, for example, requires a'small electric detonator not larger than approximately /3" diameter by /2" length to operate over a temperature range from 65 below zero to 160 above Zero Fahrenheit in response to firing energy of 200 ergs or less and with a firing time not to exceed 50 microseconds. The detonator must be adaptable to mass production, and it is further required that the electrical detonator not only be chemically stable at all humidities over the 'temjperature "range but also be capable of withstanding rough handling and accelerations to 50,000 g. It would be desirable to make the detonator even smaller and to 'reduce substantially both the firing energy and the firing time. More exacting requirements are not specified, however, because even the standard set forth cannot be met satisfactorily on a mass production basis by the types of detonators heretofore available.

One type of sensitive electric detonator heretofore em- ;ployedcomprises a bridge wire surrounded by an easily ignited mixture, sometimes referred to as a match head, which is in 'turn surrounded by, or adjacent to, a detonat- .ing explosive. The bridge wire has often been made of a diameter -on the order of .0001 or .0002. inch and in an extreme case as small as 80 microinches. It has been :made of various metals including iron, platinum, platinum alloys, and nickel chrome alloys.

One disadvantage of such a detonator is that any attempt to decrease the diameter of the bridge wire for the sake of avoiding the need for excessive current in the firing circuit is defeated "by the fact that exceedingly :fine wires are vulnerable to breakage in the course of ifabrication of the detonator. Another difficulty is that wires of material .m'ostsuited 'for the purpose of igniting the match head are not readily soldered to provide electrical connectionsgood for long periods of storage. Some of the compositions used for match heads, moreover, are subject to serious deterioration, especially when subjected to moisture and high temperatures. It is also to be .noted'that some of the match head compositions that are quite satisfactory in most respects are undesirably sensitive to impact.

Another form of electrical detonator heretofore em- "ployed for military purposes is commonly known as a graphite film bridge. .Such a detonator is formed by twist-ing'together two pieces of copper wire, for example, 22 gage copper wire, which are coated with suitable material for electrical insulation from each other. The inter-twisted wires are mo'ldedinto a plastic block, which is-then sectioned to expose two wire ends side 'by side "to serve as electrodes 'in a firing circuit. In a typical construction, the two exposed wire ends are covered first with the graphite film, then by two successive lacquer explosive component of the conductive may be, for example, pure firing circuit will be 2,918,871 Patented Dec. 29, 1959 2 coats impregnated with a relatively sensitive detonating material such as lead styphnate.

The graphite film bridge shares several of the disadvantages of the fine wire bridge including deterioration over a period of time and sensitivity to impact. Both of these types of detonators have the further disadvantage of providing a mass of metal immediately adjacent the zone of initial ignition to draw off a substantial portion of the initial heat by conduction.

The broad object of the present invention is to avoid the described disadvantages of prior art electric detonators and to meet not only the above military specification but also even more stringent requirements. 'It is another object of the invention to produce exceptionally -small detonators with higher standards of performance than heretofore attained; also to provide 'a detonator construction that is highly suitable for mass production with high quality and especially high reliability. in general, these objects are achieved by using a novel conductive mix in which electrically conductive particles, exemplified as carbon particles in the following disclosure, are dispersed in a body of detonati-ng material to provide conducting paths of microscopic cross-sectional dimension whereby an ignition temperature may be generated in the detonating body by an extremely small electric current. The idetonatin'g mix may be partly or entirely :lead .azide. The conductive mix is preferably employed adjacent a main charge which lead azide or :some other suitable material.

One object of the inventionis :to solve the general problem of providing a detonating mix having an adequate number of conductive paths of the required "character well distributedthroughoutithe mix. This general problem resolves into two separate problems, namely, :the problem of providing a conductive mix that will afford numerous microscopic conductive paths of the :required character and the problem of incorporating an appropriate .number of such paths in a firing circuit. :If too many paths are included in the firing circuit, :unduly ;high energy will be requiredto fire the mix and, on the other hand, :since the mix is not microscopically homogeneous, steps to :minimize the number of paths :in the :firing circuit are taken at the risk of making the rdetonator inoperative.

The first problem of 'providinga conductive mix with adequate microscopic conductive paths of the required character is solved .-in part in :the present .;invent-ion-:by selection of an appropriate conductive element 403? {the mix, typically amorphous carbon and preferably :a suitable form of carbon black. While channel black and the like may be employed for this purpose, a feature 0f the preferred embodiment of the invention in a detonatorr-of exceptionally small .size is the use of thermal :acetylene black. Particles of commercial thermal acetylene Ebla'ck have an average dimension of about 5.00 .A, and this form of amorphous carbon is especially desirable for the purpose of forming conductive paths because, under the microscope, these minute particles are seen .to ,be joined together in chains so that a mass of .the carbon has a three-dimensional fishnet structure.

The second problem of reliably incorporating .an .appropriately small number of these paths in the firing circuit is solved in the preferred practice of the invention byproviding one electrode of relatively large area ,for electrical contact with a relatively large number ,of the random paths and providing a second electrode having a contact area of minute dimensions "to complete the firing circuit. Thus, with uniform distribution of the paths throughout the mix, the *number of paths included in "the determined by the "smaller electro'de.

It has been found that with "reasonable care to obtain a the carbon with the detonating mathorough mixture of will vary, of course,

sure. but also by the percentage of In generaL'this percentage will usually exceed 1% but of :the conductive mix is an important factor.

.-utilization of an electrode construction which n accomplishes this particular object but also accomplishes 7 pointed configuration, to

metal. into the mass of the lead azide used alone with tion of the electrodes,

terial, the pathswill be sufficiently uniformly distributed throughout the mixture for the required high probability that an operative number of paths will be contacted by a small electrode of some given area."

It has been found desirable to compact the conductive .mix, preferably under a pressure of more than 10,000 ,p.s.i.,ifor a number of reasons. Compacting not only I promotes intimate contact between'the two electrodes and i performance.

The size of the controlling area of the smaller electrode with'the number of conductive paths exposed in a unit area of the mix, and the number of paths will be controlled not onlysby'the confining prescarbon in the mix.

will usually be less than 25%.

.Ithas also been found. that the geometryvof the small electrode surface relative to the geometry of the detonathe geometryiof the body In this respect, one object of the invention istoprovide means to serve as the smaller electrode that will have an electrode surface of a configuration to assure electrical contact with an operative number of conducting paths. A feature of the preferred practice of the invention is the not only tor as a whole and, especially,

The various objects and advantages of the invention will be apparentin the following detailed description of preferred practices of the invention considered with the accompanying drawing.

In thedrawing, which is to beregarded as merely illustrative:

Fig. l is a sectional view on a greatly enlarged scale of theinvention in the form of a detonator of small dimensions constructed to respond to exceedingly low'firing energy, the view showing the detonator at an advanced stage of the fabrication procedure;

the further objects of providing maximum utility of the generated heat and maximum freedom for ignition to spread through the detonating material.

This group of objects is attained by providing'a small elongated electrode element, preferably of tapered or I penetrate or be surrounded by the body of the conducting mix. It hasbeenfound especially advantageous to use a needle-like electrode point j to penetrate the conducting mix, apparently because such penetration, in displacing local conductive paths at the periphery of the needle point during compaction, results in intimate contact with a desirable number of path ends. Since a needle or pin point electrode is of relatively small mass, only an exceedingly small proportion of the initially generated heat is lost by conduction to or through the In addition, such a pointed electrode extending conductive mix provides maximum freedom for initial ignition to spread radially in all directions.

A further feature of the invention is the discovery that the carbon black is highly responsive to energizing current, and that the responsiveness may be increased by employing lead azide produced by precipitation in the presence of polyvinyl alcohol. A further important discovery is that the responsiveness may be increased even more by processing lead azide in advance of the mixing step in such manner as to provide fresh cleavage surfaces and/or freshly abraded surfaces "acteristics of the detonator. It will be apparent to those skilled in the art that the character of the carbon, the perthe character of the detonating pressure, the area and configuraand vother factors may be varied to modify the responsiveness and behavior of the conductive mix.

centage of the carbon, material, the confining nator includes a container Fig. 2 is a similar view showingthe finished'detonator;

Fig.3 is a transverse section taken as indicated-by the line 3-3 of Fig. 2;

Fig. 4 is a sectional view of a second embodiment of the invention.

Fig. 5 is a sectional view of a third and very satisfactory embodiment'ofthe invention after compaction but just prior to the crimping operation; and

Fig. dis a sectional view of the pre-pelleted conductive mix in its retainer ring, used in the embodiment of Fig. 5.

In the preferred practices ;of theinvention the deto providing two electrodes in- I sulated from each other for inclusion in'a firing circuit,

thefiring circuit being completed by a conductive mix, As heretofore stated, it is preferred that one of the two electrodes have a relatively large efl'ective area in contact with the mix and that the other in the container.

electrode have a relative smallcontacting area'todetermine the number of conductive paths in the firing circuit. In this regard, features of the embodiments of the invention shown in the drawing are ,the'use of the container per se, or at least a portion of thecontainer, structure or an element-therein, as therlarger electrode; and the incorporation in the container structure of asuitable member to serve as the smaller electrodes In the first form of the invention, shown in Figs. lto 3, the container structure includes a cylindrical shell 10 that is open at one end and is closed at the other end by a bottom wall 11, the lower corners being preferforce to compact the contents of the shell and that the cylindrical wall of the shell may be deformed or crimped into engagement with the walls of the groove 13, as shown at 14 in Fig. 2, to confine the enclosed material permanently under the desired pressure. The cylindrical shell 10 may be made, for example, of aluminum alloy of the type known as Alcoa 11ST3, and the plug 12 may be made'of aluminum alloy of the type known as Alcoa 61ST6, but it will be understood that other materials may be employed and that the plug 12 may, if desired, be made largely of various non-conducting materials such as glass, ceramics and various plastics.

In this particular practice of the invention; it is contemplated that the plug 12 will be provided wtih a suitable member to serve as the smaller of the two electrodes of the firing circuit. For this purpose, such a member may be of the approximate sizeand configuration of an ordinary pin. In fact, an ordinary No. 14 tinned steel bankpin having a diameter of approximately .0285" has been used successful for the smaller electrode. Preferably, however, the smaller electrode for this particular detonator is in the form of standard watchmakers thread threads per inch.

a small screw 18 having a of .0465" diameter and Such a screw may -be insulated from the plug 12 in any suitable manner and for this purpose may be subjected -to a :hard anodizing treatment to produce an aluminum oxide layer approximately .002 thick. The screw is initially threaded undersize to allow for the added thickness of the oxide layer. Preferably, a hermetic seal between the screw and the plug 12 is ,proadded by coating the threads with asuitableplastic sealing agent such as Formvar -before assembly and then curing the Formvar by baking. The installed screw 18 is in effective engagement with the plug 12 to resist the pressure of the material in the container and is effectively insulated from the plug by the surrounding nonconducting-coating 19.

.As can be seen in the screw 18 forms a pin point the interior of the container and penetrates Figs. 1 and .2, the leading end of 20 which extends into the body of material therein. -As heretofore stated, the size and configuration of the pointed portion 20 of the screw may be similar to the size and configuration of the point of an ordinary tinned pin. The length of the pin point 20 that extends into the mass of material in the container may be as .short as .010" but preferably is on the order of .025. After the screw 18 is anodized, a ,prede'termined portion of the oxide coating is removed from the extreme point of the screw to expose a portion of the metal for contact with the conductive mix inside the detonator. in this instance, for example, the exposed area at the tip of the screw may be about .003 in diam- -eter. A feature of this practice of the invention is the simplicity of controlling the effective size of the smaller .electrode by removing a predetermined amount of the insulating coating from the pin point.

With the pin point 20 extending into the interior of the container to serve as one of the two electrodes of the firing circuit, the surrounding cylindrical wall of the container may serve as the other and larger electrode. A feature of the form of the invention shown in Figs. 1-3 is the concept of making the plug 12 of hollowconfiguration so that the plug itself provides a cavity for the conductivemix and thus, in effect, "forms a portion of the circumferential wall of the container. For this purpose, the plug 12 is formed with a cavity 21 having a cylindrical wall 22, the cavity being concentric to the axis of the cylindrical shell and the pin point extending into the cavity along that axis. An advantage of this construction is that the cavity 21 may be packed with amass 25 of the conductivemix before the plug is assembled to the container.

Prior to final assembly, the cylindrical shell 10 is packed either with a mass 26 of a suitable single material to serve as the main detonator charge, or alternatively, with two layers of material contacting, for example, at a line AA, the upper to serve as the main charge and the other as the booster charge. The main charge may comprise a material such as lead azide, whereas the booster charge may comprise any suitable material such 'astetrylyPETN or RDX.

'When the detonator is prep'acked with the conductive mix entirely in the plug, the cylindrical wall 22 of the cavity 21 serves as the larger electrode in the firing circuit. If the conductive mix occupies not only the 'cavity 21 but also the upper portion of the cylindrical shell 10 adjacent the cavity, the larger electrode will also include a portion of the cylindrical shell. Preferably, however, the conductive mix is confined to the cavity 21.

The carbon-particles of the conductive mix will usually be a form of amorphous carbon and preferably a carbon black produced either by the thermal decomposition of a hydrocarbon or by partial combustion of a hydrocarbon.

In this instance, the carbon content of the conductive mix is preferably commercial thermal acetylene black such as produced by Shawinigan Chemicals, Ltd., which carbon black forms a reticulate chain structure with 'from 6 to 100 or more particles in a chain. The mean 6 particle diameter is about 4S mu, which is equivalent to 1.8 .microinches. This carbon black is supplied commercially in 'two forms designated, respectively, as 50% compression and 100% compression, the latter being preferred in this particular conductive mix.

The conductive mix may be provided by mixing the acetylene black with any suitable finely divided detonating material such as lead azide, lead styphnate, DDNP, and the like. A feature of the preferred practice of the invention is the use of lead azide alone in combination with the carbon black. While dextrinated. lead azide may be employed in some practices of the invention, in this instance a special form of lead azide known as PVA is employed, which lead azide is produced by precipitation in the presence of polyvinyl alcohol. Such a lead azide is commercially available from the Western CartridgeCompany and is in the form of microscopic prismatic crystals whose length may be 6 or 8 times their cross dimension.

It is contemplated that the carbon particles and the particles of detonating material will be mechanically intermixed to a sufficient degree to insure widespread distribution of the carbon particles throughout the mass. For limited-volume production, a slightly modified Fisher mortar-grinder has been employed successfully for this purpose. This device uses a porcelain mortar about 5 in diameter which is continuously rotated by a motor. At the same time, a suitable rubber paddle, substituted for the usual porcelain pestle, oscillates back and forth through the center of the mortar in an arc of adjustable amplitude. The device also includes a thin blade of rubber to serve as a scraper for continuously returning the material from the sides of the mortar to the bottom. It has been found satisfactory to load the mortar with approximately 500 mg. of the mixture for each batch and to mix the batch for approximately ten minutes. For large-voliune production, other mechanical mixing devices can be used.

The mixing operation, of course, damages the chainlike structure of the thermal acetylene black and therefore should not be prolonged unless it is desirable to modify the chain structure. In most practices of the invention, it is desirable to keep the chains of carbon particles relatively long, especially when responsiveness to relatively low firing energy is intended. As heretofore stated, this special lead azide may desirably be processed in advance to provide fresh cleavage surfaces and/or freshly abraded surfaces for contact with the carbon particles. For this purpose, the lead azide may be processed alone in the described mixer for approximately 5 minutes in preparation for the step of mixing the lead azide and carbon particles together.

The percentage of carbon black mixed with the detonating material may vary between 1% and 25% by weight. Satisfactory results have been attained with various intermediate proportions including l /2%, 2%, 3%, 6% and 10%. When the proportion of acetylene black approaches 25%, however, the behavior of the mixture may tend to become inconsistent and the carbon may tend to form minute balls containing the lead azide particles. In the particular embodiment of the invention shown in Figs. 1 to 3, where responsiveness to relatively low firing energy is sought, the acetylene black preferably comprises about 3% of the mixture.

The following is an example of an assembly procedure and the types of detonating charges used: After the screw 18 has been inserted in the plug 12 with a portion of the pin point 20 of the screw exposed to serve as the smaller electrode, the conductive mix is packed or compac-ted into the plug cavity 21 as a separate operation. Into the open cylindrical shell 10 are placed first a booster charge of mg. tetryl, then a main charge of 124 mg. lead azide, following which these two layers are compressed. The plug 12 with the conductive mix therein is then inserted into the cylindrical shell 10 as shown in .7 Fig. 1 and is forced downward to compact the contents of the container to the desired degree. The cylindrical shell is then crimped into the groove 13 of the plug to confine the contents permanently under the desired pressure. The nature of the booster charge material, if used, may be varied, depending on the output characteristics required of the detonator.

Various magnitudes of pressure may be used in various practices of the invention for compaction both of the conductive mix and also the main and booster charges, if the latter is used. Usually these pressures of compaction will exceed 10,000 p.s.i., especially in detonators larger than those previously mentioned dimension ally. Compaction pressures of 15,000 to 20,000 p.s.i. have been used with good results, and the pressures may be 40,000 p.s.i. or even higher than 60,000 p.s.i. if desired. In one satisfactory practice of the invention, the conductive mix 25 is packed into the cavity 21 at 18,000 to 20,000 p.s.i. and the plug is installed to place the main charge under a pressure of approximately 25,000 p.s.i. In another and sometimes preferred practice of the invention, the plug cavity is preferably loaded at about 45,000 p.s.i. prior to compressing the main charge under approximately 20,000 p.s.i., the result being excellent with respect to stability and sensitivity of the detonator.

A number of factors contribute to the successful operation of the described detonator with exceptionally low firing energy. The preliminary processing of the lead azide to provide fresh surfaces for the carbon is one factor. The use of thermal acetylene black to provide the reticulated chain structure is another factor. The limitation of the mixing time to no more than about 10 minutes to minimize damage to the carbon chain structure is often important. The compacting of the conductive mix, as well as the pressure contact between the conductive mix and the charge, is of some importance in forcing the carbon particles together to form conductive paths and in assuring uniformity of performance. The pressure contacts among the carbon particles also reduce resistance to current flow along the conductive paths. Along with these factors, the use of a mix comprising 3% thermal acetylene black in combination with a pin point electrode having an eifective point area of approximately .003" in diameter accounts for the inclusion of a usually-appropriate number of the conductive paths in the firing circuit.

In this regard, it is a significant fact that if the inner end of the screw 18 is cut off flush with the end wall of the cavity 21 and an area of .003" or larger at the end of the screw is exposed to serve as an electrode, the firing energy must be increased substantially above the level required with the pointed screw. Apparently, the pin point 20 Wedges into the body of the conductive mix with consequent improved contact between the metal of the screw and the carbon paths through the conductive mix.

The advantages of the described conductive mix may be appreciated by comparison with the fine wire used in bridge type detonators. The minimum practical diameter of the fine wire bridge is approximately .0001" which is approximately 55 times the diameter of a chain of thermal acetylene black and, on the basis of crosssectional area, this ratio is squared with a resultant advantage of approximately 3,000 to 1. In addition, the lower density and lower specific heat of carbon compared to metals provide an advantage of 2.5 so that a bridge consisting of a single chain of acetylene black should be approximately 7,500 times as sensitive as a fine wire bridge. Of course, this advantage is not fully realized because a conductive mix necessarily provides many conducting paths connected in parallel in the firing circuit. The method of fabricating the detonator, however, With special reference to the minute pin point electrode, sufiiciently limits the number of conductive paths provide sensitivity far exceeding the sensitivity attained with a fine Wire bridge. Distribution of the carbon paths throughout the conductive mix is not microscopically'uniform, but, nevertheless, is adequate to provide the necessary high probability that an operative number of the conductive paths will be contacted by the pin point electrode.

The described detonator construction makes it possible to meet more strict requirements than set forth in the previously-mentioned military specification for a small detonator. The military specification requires a firing energy as low as 200 ergs, or less, whereas the described detonator operating witha reliability of 99.6% responds to a firing energy of less than 50 ergs. In fact, the de scribed detonator will operate at 5 ergs, and successful tests have been made with firing energy less than 1 erg. The military specification requires that the firing time not exceed 50 microseconds, whereas the described detonator operates with a time delay of 10 microseconds or less. The military specification requires a maximum length of 0.5" and a maximum diameter of 0.3", but all of the requirements may be met by a detonator of the present construction having a maximum length of 0.4" and a maximum diameter of 0.19".

In the second form of the invention, shown in Fig. 4, the container structure comprises a cylindrical container 30 closed by a suitable plug structure including a plug 31. In the construction shown, the plug 31 has a body 32 of circular cross section to fit inside the cylindrical container and is formed with an annular shoulder 33 tapering to a straight stem 34. All of the plug 31 except the outer end is coated with insulating material, for example, an aluminum oxide coating 35 produced by a hard anodizing treatment. i

The plug 31 is formed with a smalleror pin point electrode 36 extending concentrically from'itsinner face, and an area of the coating of approximately .003" in diameter is removed from the pin point to provide the smaller firing electrode as heretofore described.

A main charge 37 is loaded into the container 30, supplemented if desired by a booster charge. A top layer 38 of the conductive mix is then added. The plug 31 is then inserted into the container 30 and pressed inwardly to subject the conductive mix and the charge to the desired pressure, preferably a pressure above 10,000 p.s.i., and then the open end of thecontainer 30 is crimped over the tapered shoulder 33 of the plug, as in dicated, during the compaction of the contents to confine the contents permanently under the applied pressure. With the same ingredients prepared in the same manner as heretofore described, the detonator shownin Fig. 4 will operate with the same elficiency as the detonator shown in Figs. 1 to 3. It will be noted that in Fig. 4 the larger electrode in the firing circuit comprises a wall of the container proper, the plug serving as the smaller electrode. t

The alternative embodiment of Figs. 5 and. 6 is often the preferred one for many reasons. Here the container structure includes a closed-bottom cylindrical container 40, a closure plug 41 and a mix-retaining ring 42 which fits snugly into the container to be in good electrical contact therewith. The plug 41 and the ring 42 form a plug structure 43 capable of compressing the contents of the container 40 in a manner similar to that described for the plug structure of Figs. 1 and 2. A mass 44 of the conductive mix is compressed in the central space of the ring, before insertion of the latter in the container 40, to contact the peripheral surface 45 of this central space. The conductive mix is thus pre-pelleted into the ring 42, preferably with the exposed surfaces of the mass 44 bulged slightly to form convex surfaces, the crests of which are respectively a few thousandths of an inch, typically in the firing circuit to about 0.002", beyond the corresponding plane surface not always necessary in pracof the ring. This feature,

act-asst tic'e, assists in insuring good contact with the firing electrode and the charge during final assembly.

The plug 41 has a body =47 of circular cross section having a cylindrical face "48 containing a circumferential groove 49. 'It may have a sm'aller stem or neck 50 and, as in the embodiment of Fig. 4, is coated with an insulating material, typically an aluminum oxide coating 51pmduced by an anodizing treatment, the coating being removed to expose the tip of 'a pin point or smaller electrode'SZ of the general size previously discussed.

-A main charge, supplemented if desired by a booster charge, is placed in the container 40 with or without initial compaction. The :pelleted ring 42 is then inserted and theplug 41 forced downward to press the pelleted ring 42 against the charge in compacting relationship, the ele ments now being in the position shown in Fig. 5. The wall of the container 40 opposite the circumferential groove 49 is now deformed or crimped into the latter, as suggested by the dotted lines 53 of Fig. 5, "to retain the contents under pressure.

The manner of sealing or maintaining the contents under pressure by deforming or crimping a portion of the container into a groove of the plug isa cheap and efficient means for accomplishing the desired result. However, the inventionis not limited thereto as various alternatives can be used. I

It will be apparent to those skilled in the art that the sensitivity of the detonator may be lowered, if desired, by various departures from the foregoing descriptions. Increasing the carbon content of the conductive mix, for example, will increase the number of conducting paths in the firing circuit with consequent increase in therequired firing energy. The application of less pressure to the contents of the detonator may also'have some effect in increasing the resistance to current flow along the conductive "paths with consequent increase in the required level of firing energy. Increasing the area of the pin point that is exposed to the conductive mix will also increase the 'numberof conductive paths in the firing cir- 'cu"" ivithconsequent increase in the firing energy and, -'as heretofore noted, substituting a flat electrode for the pin point electrode also increases the required firing energy. The detonator may also be made' less sensitiveby omitting the preliniinary processing of the lead azide and by using "other form's of lead az ide or other detonating materials.

The'fiexibility of the invention may "be illustrated by considering'a special 'military requirement fora detonato-r to be-fired from a condenser rated at .08 micro'farad and charged to 60 volts and not to fire in response to a 20-volt charge "on the "same condenser. It has been found that -the sensitivity "can be reduced to the desired degree by using a conductive consisting of dextrinated lead azide containing of R-40 channel black available commercially from Continental Carbon Company. The contents of the detonator compacted under 20,000 ,p.s.i. behaves as desired, but increasing the pressure to 40,000 ,p.s'.i. makes the detonator 'too sensitive.

The ,preferred practices of the invention specifically described .hereinby way of illustration and disclosure will suggest to those skilled in theart various changes, modifications and other departures that properly lie within the spirit and scopeof the appended claims. For example, -in applications not involving high acceleration of the detonator, the conductive mix and/or the main and booster charges may be pressurized or otherwise com- ;=pacted in advance to produce bodies in tablet form for insertion into the detonator container with little or no f-pressure application in the final fabrication step. Also, while thepreviousdescription has largely exemplified carbon particles as the conductive material of the mix, the invention is 'not :limited thereto, :particularly inthose com- :mercial or non-military applications where lower sensiitivities are'tolerable. "Thus, the invention can betformudated by use of various conductive powders, other than Em carbon, whether :metallic or nommet'allic, usuallywithin the'ranges-hereinbefore set forth.

' l cl'aim as my invention:

1. A imethod of fabricating an electrical detonator able of being fire'd by current .ilow through 'a firing circuit including 'two spaced electrodes one of which is a penetrating-type electrode, which method includes the steps of: "forming a mixture of finely divided detonating material and particles of an electrically conductive material; introducing a mass of such mixture into the zone of the spaced electrodes; compacting such mass of the mixture against the penetrating electrode "and in bridging contact with said spaced *electrodes'to form a compacted mass with numerous electrically conductive paths therethrough; and controlling the electrical detonation sensitivity by controlling the degree of compaction of said mass against said penetrating electrode.

2. A method of fabricating an electrical deto atorcapable of being fired by current flow through a firin'g circuit including two :spaced electrodes respectively bounding Ispaced portions of a zone, one of which electrodes is a pin with an exposedpoint, which method includes the steps of: forming a mixture of finely divided detonating material and particles of an electrically conductive material; introducing a mass of such mixture into said zone and compacting such mass of "the mixture against and around said exposed point and in bridging contact with said spaced electrodes by relative movement of said electrodes in a direction decreasing the volume'of said zone to form a comp-acted mass with numerous electrically conductive paths therethrough "radiating from said exposed point.

3. A method of fabricating an electrical deton'ato'r 'c'a'pable of being fired by current-flow through a firing circuit including two spaced electrodes having exposed surfaces of widely different al-ea, which method includes the steps of: producing :a mixture "formed exclusively of particles, said mixture comprising discrete finely divided particles of amorphous carbon and finely divided particles of a detonating material, said 'detona'ting material comprising particles of lead azide; processing the lead azide particles immediately prior to mixing same with said amorphous carbon to form 'fr esh surfaces on said lead azide particles by cleavage and abrasion; bringing a mass of said mixture into the-zone o'f s'aid electrodes; and compacting said' ma'ss -aga'inst safid exposed surfaces with sufficient pressure to bring the'discrete finely divided particles of carbon into electrical contact to form conductive 'paths extending through said mass from the electrode of small exposed area.

'4. A method of fabricating anelectrical detonator hav- 'ing a first electrode of relatively large surface area and a second electrode of relatively small'surface area, said detonator being capable of being fired by current flow through a 'firin'g circuit including said electrodes, which method includes the steps of: forming a mixture of finely divided de'ton'ating material and particles of an "electrically conductive material, s'aid particles forming electrically conductive paths through the 'mixture;'-placing a mass -.of2the'mixture in contact=with both said first electrode of relatively large :su'rface'area and said second electrode of 'relatively'small=surface'area; and relatively moving said mass and said second electrode'to compact said mass at the point of contact with said second electrode of relatively small surface area, said mass providing nu- .merous ofsaid electricallyconductive paths therethrough,

, said first electrode providing contact with a relatively large 'group of said-eonductivepaths and saidsecond electrodeiproviding contact with .a relatively small proportion ofisaid group of paths.

"5 Amethod of fabricating a 'prir'nin'g composition for "an electrical detonator, which method includes thesteps of: agitating a quantity of finelydivi'ded -lead azide :particl'es to forrn fres'h surfaces thereon by cleavage and abrasion; and :mixing the resulting particles -with finely I surfaces,

non-conductive of electricity;

11 divided particles of carbon for contactwith said fresh said carbon forming electrically conductive paths through the lead azide:for ignition of said lead a'zide upon current fio'w through some of saidpaths.

6. A method of fabricating an electrical detonator having electrodes adapted for connection. to a firing circuit, said electrodes including a-first electrode, which method includes the steps of: coating at least a portion of the surface of said first electrode with a material-relatively locally removing a portion of said coating to expose a relatively small electrode area; and compacting a mixture'of finely divided detonating material and finely divided electrically conductive material against said exposed relatively small electrode area.

7. An electrical detonator having in combination: a pair of electrodes adapted'for connection in a firing cir cult; and a conductive mixture interconnecting said electrodes, said mixture comprising a finely divided electrically conductive material intermixed with particles of a detonating material, one of said electrodesbeing a pointed electrode. providing a relatively smallexposed area extending into said conductive mixture, the other of said electrodes providing a relatively large exposed area in contact with said conductive mixture.

7 8; A detonator as defined in claim 7 in which said electrically conductive material is amorphous carbon.

amorphous carbon is carbon black. 1

1,0. A detonator as defined in claim 8 in which said amorphous carbon is thermal acetylene black.

11. A detonator as defined in claim 8 in which said amorphous carbon is channel black. 12. A detonator as defined in claim 7 in which said 9. A detonator as defined in claim 8 in which said detonating material consists at least in part of lead azide.

' 13. An electrical detonator including in combination:

a container adapted to receive an explosive charge a ring havinga central space bounded by a wall and having a conductive mix of discrete single particlesof detonating material and discrete particles of finely divided carbon compacted in said central space, said wallforming a first electrode surface of relatively large area in contact with said conductive mix, said ring being in said container; a second metallic electrode providing a sharp pointpenetrating said conductive mix and having an extremely small exposed area localized at said point in electrical contact with said conductive mix at a position within said central space but spaced from said wall; means for electrically insulating said electrode surfaces from each other; and a closure for said container.

14. An electrical detonator having in combination: a closed container structure containing a mass of a conductive mixture compacted under a pressure of at least 10,000 p.s.i., said conductive mixture comprising finely divided carbon black intermixed with particles of detonating material, at least a part of said container structure being made of metal and being in contact with said mass to serve as one electrode in a firing circuit; a metal member providing a pointed portion having a small surface extending into said compacted mass to lie in contact therewith and serve as a second electrode in such firing circuit; and means for electrically insulating said metal member from said metal of said container structure.

15. An electrical detonator having in combination: a completely closed container structure containing a compacted mass of a conductive detonatable mixture, said container structure being formed of a container and plug therefor, said container and plug being interlocked to retain said mass compacted, said container structure providing a first relatively large electrode surface in contact with said compacted mass to serve as one electrode in a firing circuit; a second electrode comprisinga small pin carried by said plug and having a pin point surface of extremely small area measured in millionths of a square inch exposed to and in contact with said compacted mass j of a detonating mixture in said cavity and comprising a of a detonating mixture in said cavity and comprising a granular mixture of finely divided carbon particles and finely divided particles of detonating material, said cavity providing a wall contacting said compacted mass and acting as a first electrode; a second metal member having a pin point second electrode of relatively small surface exposed in said cavity and contacting said compacted mass; and means for electrically insulating said metal members from each other."

17. An electrical detonator including in combination: a first metal member having a cavity; a compacted mass mixture of finely divided carbon and finely divided detonatingmaterial, said'cavity providing a wall contacting said compacted mass and acting as afirst electrode; a I second metal member threaded through said. first metal member and having a. relatively small surface exposed in said cavity and contacting said compacted mass to .act.

as a second electrode; and means for electrically insulating said metal members from each other, said means including a material of low electrical conductivity lying between the threads of said members.

18. An electrical detonator including in combination:

a first metal member having'a cavity;- a compacted mass of a detonating mixture in said cavity and comprising a mixture of finely divided carbon and finely divided deto' nating material, said. cavity providing a wall contacting said compacted mass and acting as a first electrode; and a second metal member having a relatively small surface .exposedinsaid cavity and contacting said compacted ,mass to act as a second electrode, said second metal member fitting said first metal member and being anodized to provide acoating-electrically insulating said metal members from each other, said relatively small surface being freeof such coating, said coating separating said metal members.

19. An electrical detonator including: a pair of electrodes adapted for connection in a firing circuit, one of said electrodes providing a relatively large surface and the other of said electrodes being the exposed end of a metallic pin, said exposed end being no larger than about .003 inches in diameter; and a conductive mixture compacted against said electrodes, said mixture comprising particles of a finely divided electrically conductive material intermixed with particles of a detonating material, said exposed end of the other of said electrodes extending into said conductive mix a distance of about .0l-.02 5 inch.

20. An electrical detonator including: metal walls defining a chamber closed at one end and open at the other; an electrically conductive closure member occupying the open end of said chamber having a small projection extending into said chamber; a coating of insulating material on said closure member completely separating same from said metal walls and completely covering all portions of said closure member facing said chamber except said projection; and a compacted mixture interconnecting said projection and said metal walls, said mixture comprising particles of a finely divided electrically chamber, the other of said electrodes being the end of a metal pin protruding from a wall of said chamber, the protruding portion of said pin being covered with .a coating of insulating material with the exception of an extremely minute exposed facing corresponding portions of said chamber; one of said electrodes having an electrically conductive relatively large exposed area bounding a portion of said chamber, the other of said electrodes having a portion projecting into said chamber, said projecting portion having an extremely minute surface area measured in millionths of a square inch, the ratio of said exposed areas being many hundred to one; means for electrically insulating said electrodes from each other; and an explosive material compacted in and completely filling said chamber, said material including a detonatable mixture of discrete single particles of a detonating material and discrete particles of amorphous carbon, said explosive material bridging said electrodes.

References Cited in the file of this patent UNITED STATES PATENTS 319,628 Russell June 9, 1885 2,086,548 Handforth July 13, 1937 FOREIGN PATENTS 16,963 Great Britain of 1912 578,300 Great Britain June 24, 1946 

